1. Field of the Invention
The present invention relates to a device for coding a picture signal by compression and, more particularly, to a picture signal compression coding device which maintains the amount of compressed data representative of a picture constant at all times.
2. Description of the Related Art
Digital picture data representative of a picture picked up by an electronic still camera, for example, are stored in a memory. Various kinds of compression coding schemes have been proposed to reduce the amount of such digital picture data and thereby the required memory capacity. Among them, a two-dimensional orthogonal transform coding scheme is extensively used because it codes data by a large compression ratio and because it allows a minimum of picture distortions particular to coding to occur.
Two-dimensional orthogonal transform coding is such that picture data representative of a single picture are divided into a plurality of blocks, and the picture data are subjected to two-dimensional orthogonal transform block by block. The picture data, which undergo the orthogonal transform, i.e., transform coefficients, are compared with a predetermined threshold so as to discard those transform coefficients which are lower than the threshold. The transform coefficients lower than the threshold are treated as data zero thereafter. The remaining data are divided by a predetermined quantizing step value, or normalizing coefficient, and thereby quantized or normalized by a step size. By this kind of procedure, the values of transform coefficients, i.e., the dynamic range of amplitudes is suppressed.
The comparison of transform coefficients with a threshold and the normalization are often executed at the same time. Specifically, when transform coefficients are normalized by a predetermined normalizing coefficient and the results are turned to integers, the transform coefficients which are smaller than the normalizing coefficient turn out to be zero.
The two-dimensional orthogonal transform coding procedure stated above has some problems left unsolved. Specifically, coding picture data by applying a predetermined normalizing coefficient to transform coefficients results in the amount of data which is different from one coded picture data to another. Such coded picture data cannot be written in a memory whose capacity is limited without resorting to troublesome processing. More specifically, when a predetermined normalizing coefficient is used, picture data containing many high frequency components will have a large amount of data when coded while picture data containing many low frequency components will have a small amount of data when coded. The coded data of the picture data in which high frequency components are predominant sometimes amounts to five to ten times the coded data of the picture data in which low frequency components are predominant.
In light of the above description, there has been proposed a system which calculates the degree to which high frequency components are predominant, i.e., an activity block by block and selects a normalizing coefficient in matching relation to the calculated activity. However, changing the normalizing coefficient and, therefore, the compressing condition from one block to another as mentioned brings about another problem that the picture quality differs from one block to another, resulting in unstable picture quality. Further, the transform coefficients have to be calculated block by block.